fix environment parsing issue when no environment has been provided & add a cleanup stage to remove the cleanup steps the latest user-defined steps

[mhewedy]

Thanks for the amazing yet simple yaml pipeline on top of jenkines!

I found this issue and I am happy to send a PR, it contains the following:

• fix the issue of passing empty string instead of an array in case of empty environment array. this issue causes the base var to break if no environment was set
• add a cleanup stage to remove the cleanup steps the latest user-defined steps